1. Field of the Invention
The present invention relates generally to a signal processing system for performing period-to-voltage conversion for converting rectangular pulse train form signal into a voltage signal having a signal level variable depending upon a period of the rectangular pulses. More specifically, the invention relates to a signal processing technology for avoiding influence of noise in period-to-voltage conversion.
2. Description of the Background Art
In the automotive suspension technologies, there have been proposed various suspension control systems which can control suspension characteristics for damping and for absorbing vibration energy for achieving both riding comfort and driving stability. In order to accomplish this, it is essential to precisely monitor the relative displacement between the vehicle bodies and the road wheels rotatably supported by means of suspension members, such as suspension arms, suspension links and so forth. Some of the sensors employed for monitoring the relative distance between the vehicle bodies and the road wheels are designed to produce signals having pulse widths which are variable depending upon the monitored distances. When such type of sensor is employed in a suspension control system, it becomes necessary to provide a period-to-voltage converter circuit for converting the relative distance indicative pulse period into a voltage signal for indicating the data of relative distance by signal voltage level.
For example, Japanese Patent First (unexamined) Publications (Tokkai) Showa 53-87276 and 56-57961 disclose signal processing systems for performing the period-to-voltage operation. The system disclosed in Tokkai Showa 53-87276, is provided with a wave shaping circuit for shaping an input signal into a constant level rectangular pulse train. The period of each rectangular pulse is detected by a period detector circuit. The period detector circuit detects the leading edge of the rectangular pulse to produce a pulse signal. A saw tooth signal generator receives the pulse signal generated by the period detector circuit to be reset and generates a saw tooth wave signal having a peak level variable depending upon the resetting interval. The saw tooth wave signal of the saw tooth signal generator is fed to a sample/hold circuit which samples the saw tooth signal and holds the saw tooth signal level at the occurrence of the reset pulse of the period detector circuit. The output of the sample/hold circuit is fed to a low-pass filter which removes a ripple contained in the sample/hold output. The low-pass filter output is fed as the period-to-voltage converted output.
The period detector circuit normally comprises a resistor, a capacitor, a diode and logic circuit. On the other hand, the saw tooth generator circuit comprises a bootstrap circuit and the sample/hold circuit comprises an input pulse synchronization type sample/hold circuit. Such circuit construction encounters the problems set out herebelow.
The saw tooth signal generator circuit causes an abrupt voltage drop as a result of the reset by the reset pulse which causes an abrupt variation of current and the creation of noise. The noise created by the saw tooth signal generator circuit tends to be propagated to associated circuits to cause malfunction of the associated circuit components in the worst case. Furthermore, the saw tooth signal generator and the sample/hold circuit has to be accurately synchronized to each other to obtain an accurate conversion operation. This clearly requires a timing pulse and thus requires a clock generator which is capable of generating a synchronization pulse. The necessity of an additional clock or synchronization pulse generator necessarily causes an increase in the cost.
On the other hand, Tokkai Showa 56-57961 discloses a period-to-voltage converter circuit which converts the rectangular wave form input signal into saw tooth wave signal. The saw tooth wave signal is converted into a direct current signal by an averaging circuit. This circuit also has the drawback of generating an OFF-set noise which is contained in the former publication.